1. Field of the Invention
The present invention relates to a battery charger and particularly to a battery charger which includes a circuit for detecting a battery voltage during a charging operation and which is operable to perform a control for stopping supply of a charging current based on the detected battery voltage.
2. Description of the Prior Art
Generally, when a battery is charged, a battery voltage increases with the proceeding of charge as will been seen from a characteristic line 80 shown in FIG. 5. The battery voltage becomes maximum when the battery has been fully charged as indicated by a point 81 in FIG. 5. When the charging operation is further continued, the battery voltage is decreased as a line part 82. Such a characteristic is utilized to stop the supply of the charging current when the battery has been fully charged, and a method called "-.DELTA.V method" is well known.
With the -.DELTA.V method, the battery voltage is detected during the charging operation so as to determine the timing when the battery voltage has been decreased by a voltage of -.DELTA.V from a peak voltage, and the charging operation is stopped at this timing. In order to ensure that this method is reliably performed, it is necessary to exactly detect the battery voltage.
FIG. 6(A) shows a circuit configuration which is normally used for detecting a battery voltage VD. In FIG. 6(A), the battery voltage VD is divided by a pair of resisters RE and RC to obtain a divided voltage VC based on which the battery voltage VD is determined. Thus, in this circuit, the battery voltage VD is proportional to the divided voltage VC as a characteristic line Ap shown in FIG. 6(C). Here, a diode Dp and a constant voltage VB serve to prevent the divided voltage VC from having a value more than the voltage VB.
As shown in FIG. 5, the battery voltage VD is never reduced to zero but maintains a value equal to or greater than a certain value even if the battery is one to be recharged. When the battery is charged, the battery voltage VD increases from such a value equal to or greater than the certain value. The characteristic line 80 shown in FIG. 5 is that obtained in case of charging of a battery having a higher rating voltage. In this case, the battery voltage varies within a range between a voltage VHB and a voltage VHP. A characteristic line 84 is obtained in case of charging of a battery having a middle rating voltage, and a characteristic line 86 is obtained in case of a battery having a lower rating voltage. Voltages VHB, VMB and VLB correspond to the base voltage for the batteries having the higher, the middle and the lower rating voltages, respectively, and voltages VHP, VMP and VLP correspond to the peak voltage for the batteries having the higher, the middle and the lower rating voltages, respectively. When these three kinds of batteries are to be charged by a common battery charger, the battery charger should include a circuit for detecting the battery voltage between VB(VLB) and VP(VHP).
It is preferable that the divided voltage VC varies by a greater extent in response to the variation of the battery voltage between VB and VP or varies along a line such as a characteristic line D (line part D1+line part D2) shown in FIG. 6(C).
In order to vary the characteristic line Ap shown in FIG. 6(C) to approach the characteristic line D (D1+D2), another circuit is proposed as shown in FIG. 6(B). In this circuit, a zener diode VZ having a breakdown voltage equal to VB is incorporated, so that the divided voltage VC is zero when the battery voltage VD is lower than the voltage VB. On the other hand, when the battery voltage VD is greater than the voltage VB, the current flows across the zener diode VZ and the resistor RC resulting in increase in the divided voltage VC.
With the latter circuit, however, the current flows across the zener diode VZ even if the battery voltage VD is lower than the breakdown voltage VB. For this reason, the divided voltage VC varies with the battery voltage VD as a characteristic line Bp shown in FIG. 6(C). Therefore, the latter circuit is not sufficient to cope with the requirement of the greater variation of the divided voltage VC in response to the variation in the battery voltage between VB and VP. Particularly, in case of charging of a battery having battery cells connected in series with each other, although the battery voltage has a value between VB and VP in a normal condition of the battery, the battery voltage may have a value lower than VB when any one of the battery cells is shorted. Further, in this case, if the characteristic line D is obtained, it is easy to detect the short circuit of the battery cell since the divided voltage VC is zero. On the other hand, if the characteristic line Bp is obtained, the divided voltage VC is not zero, so that it is difficult to detect the short circuit. Thus, the characteristic line Bp is still not sufficient although it improves the sensitivity of detection to some extent compared with the characteristic line Ap.
The present invention is therefore intended to obtain the characteristic of the divided voltage to vary substantially along the characteristic line D so as to dissolve the disadvantage of the conventional battery charger.